This invention relates to a method of preparing polycarbonates using an extruder to convert component monomers, ester substituted diaryl carbonates and dihydroxyaromatic compounds, into product polycarbonates. The invention further relates to the preparation of polycarbonates in which a precursor polycarbonate comprising ester-substituted phenoxy endgroups is subjected to extrusion to produce a polycarbonate having a higher molecular weight. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products and possessing a high level of endcapping.
Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.
The interfacial method for making polycarbonate has several inherent disadvantages. First it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.
The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer""s flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight.
Some years ago, it was reported in U.S. Pat. No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterification catalyst in the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration.
It would be desirable, therefore, to minimize the amount of catalyst required in the melt preparation of polycarbonate from bisphenols and ester substituted diaryl carbonates such as bis-methyl salicyl carbonate (BMSC). In addition, it would be desirable to provide a method for the melt preparation of polycarbonate using simple melt mixing equipment such as an extruder.
The present invention provides a method for the preparation of polycarbonate comprising extruding at one or more temperatures in a temperature range and at one or more screw speeds in a screw speed range, at least one starting material selected from the group consisting of
(A) a mixture comprising an ester-substituted diaryl carbonate, a transesterification catalyst and at least one dihydroxy aromatic compound; and
(B) at least one precursor polycarbonate comprising ester-substituted phenoxy terminal groups.
The present invention further relates to a single step method for preparing highly endcapped, polycarbonates having very low levels of Fries rearrangement products, said polycarbonates comprising ester substituted phenoxy endgroups.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy aromatic compound.
As used herein the term xe2x80x9cprecursor polycarbonatexe2x80x9d refers to a polycarbonate which when subjected to extrusion in the presence of a transesterification catalyst affords a polycarbonate having a higher molecular weight after the extrusion than before it.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.
xe2x80x9cCatalyst systemxe2x80x9d as used herein refers to the catalyst or catalysts that catalyze the transesterification of the bisphenol with the diaryl carbonate in the melt process.
The terms xe2x80x9cbisphenolxe2x80x9d, xe2x80x9cdiphenolxe2x80x9d and xe2x80x9cdihydric phenolxe2x80x9d as used herein are synonymous.
xe2x80x9cCatalytically effective amountxe2x80x9d refers to the amount of the catalyst at which catalytic performance is exhibited.
As used herein the term xe2x80x9cFries productxe2x80x9d is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product comprises structure VIII below, which affords 2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate.
The terms xe2x80x9cFries productxe2x80x9d and xe2x80x9cFries groupxe2x80x9d are used interchangeably herein.
The terms xe2x80x9cFries reactionxe2x80x9d and xe2x80x9cFries rearrangementxe2x80x9d are used interchangeably herein.
The terms xe2x80x9cdouble screw extruderxe2x80x9d and xe2x80x9ctwin screw extruderxe2x80x9d are used interchangeably herein.
As used herein the term xe2x80x9cmonofunctional phenolxe2x80x9d means a phenol comprising a single reactive hydroxy group.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclcopropyl, cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.
According to the method of the present invention, extruding a starting material (A), a mixture comprising an ester-substituted diaryl carbonate, a transesterification catalyst and at least one dihydroxy aromatic compound; or starting material (B), at least one precursor polycarbonate comprising ester-substituted phenoxy terminal groups; affords a product polycarbonate. In some instances the method according to the present invention employs both starting materials (A) and (B), as where an ester-substituted diaryl carbonate, a dihydroxy aromatic compound and a transesterification catalyst are first partially reacted to form a mixture comprising said ester-substituted diaryl carbonate, a dihydroxy aromatic compound and a transesterification catalyst as well a precursor polycarbonate comprising ester-substituted phenoxy endgroups, and said mixture is then extruded.
In one aspect the of the present invention the product polycarbonate is prepared by introducing an ester substituted diaryl carbonate, at least one dihydroxy aromatic compound, and a transesterification catalyst into an extruder to form a molten mixture in which reaction between carbonate groups and hydroxyl groups occurs giving rise to polycarbonate product and ester-substituted phenol by-product. The extruder may be equipped with vacuum vents which serve to remove the ester-substituted phenol by-product and thus drive the polymerization reaction toward completion. The molecular weight of the polycarbonate product may be controlled by controlling, among other factors, the feed rate of the reactants, the type of extruder, the extruder screw design and configuration, the residence time in the extruder, the reaction temperature and the number of vacuum vents present on the extruder and the pressure at which said vacuum vents are operated. The molecular weight of the polycarbonate product may also depend upon the structures of the reactant ester-substituted diaryl carbonate, dihydroxy aromatic compound, and transesterification catalyst employed.
The ester-substituted diaryl carbonates according to the present invention include diaryl carbonates having structure I 
wherein R1 is independently at each occurrence C1-C20 alkyl radical, 2 i C4-C20 cycloalkyl radical or C4-C20 aromatic radical, R is independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, C4-C20 aromatic radical, C1-C20 alkoxy radical, C4-C20 cycloalkoxy radical, C4-C20 aryloxy radical, C1-C20 alkylthio radical, C4-C20 cycloalkylthio radical, C4-C20 arylthio radical, C1-C20 alkylsulfinyl radical, C4-C20 cycloalkylsulfinyl radical, C4-C20 arylsulfinyl radical, C1-C20 alkylsulfonyl radical, C4-C20 cycloalkylsulfonyl radical, C4-C20 arylsulfonyl radical, C1-C20 alkoxycarbonyl radical, C4-C20 cycloalkoxycarbonyl radical, C4-C20 aryloxycarbonyl radical, C2-C60 alkylamino radical, C6-C60 cycloalkylamino radical, C5-C60 arylamino radical, C1-C40 alkylaminocarbonyl radical, C4-C40 cycloalkylaminocarbonyl radical, C4-C40 arylaminocarbonyl radical, and C1-C20 acylamino radical; and b is independently at each occurrence an integer 0-4.
Ester-substituted diaryl carbonates I are exemplified by bis-methyl salicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicyl carbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate, bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate and the like. Typically bis-methyl salicyl carbonate is preferred.
The dihydroxy aromatic compounds according to the present invention include bisphenols having structure II 
wherein R3-R10 are independently a hydrogen atom, halogen atom, nitro group, cyano group, C1-C20 alkyl radical C4-C20 cycloalkyl radical, or C6-C20 aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO2 group, a C1-C20 aliphatic radical, a C6-C20 aromatic radical, a C6-C20 cycloaliphatic radical or the group, 
wherein R11 and R12 are independently a hydrogen atom, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C4-C20 aryl radical; or R11 and R12 together form a C4-C20 cycloaliphatic ring which is optionally substituted by one or more C1-C20 alkyl, C6-C20 aryl, C5.C2, aralkyl, C5-C20 cycloalkyl groups or a combination thereof.
Suitable bisphenols II are illustrated by 2,2-bis(4-hydroxyphenyl)propane (bisphenol A); 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 2,2-bis(3-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane; 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-chloro-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-disopropyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-di-t-butyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-diphenyl-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)propane; 2,2-bis(4-hydroxy-2,3,5 ,6-tetrabromophenyl)propane; 2,2-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)propane; 2,2-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)propane; 2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)cyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)cyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-chloro-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)cyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl )cyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-3-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dichloro-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dibromo-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-methylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-chloro-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-4-hydroxy-5-isopropylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-t-butyl-5-chloro-4-hydroxy phenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; bis(3-chloro-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3-bromo-5-phenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-disopropyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-di-t-butyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(3,5-diphenyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrachlorophenyl) -3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetrabromophenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(4-hydroxy-2,3,5,6-tetramethylphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dichloro-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 1,1-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; 4,4-dihydroxy-1,1-biphenyl; 4,4xe2x80x2-dihydroxy-3, 3xe2x80x2-dimethyl-1,1-biphenyl; 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dioctyl-1,1-biphenyl; 4,4xe2x80x2-dihydroxydiphenylether; 4,4xe2x80x2-dihydroxydiphenylthioether; 1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene; 1,3-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene; 1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene and 1,4-bis(2-(4-hydroxy-3-methylphenyl)-2-propyl)benzene. Bisphenol A is preferred.
The polycarbonate prepared according to the method of the present invention comprises ester substituted phenoxy endgroups having structure III 
wherein R1 and R2 are defined as in structure I and b is an integer 0-4; or endgroups derived from structure III, for example, endgroups introduced by displacement of an ester substituted phenoxy endgroup having structure III by a monofunctional phenol such as p-cumylphenol. In one embodiment of the present invention structure III is the methyl salicyl group IV. The methyl salicyl endgroup IV 
is preferred.
The present invention a provides a method for the preparation of polycarbonate, said method comprising extruding at least one starting material selected from the group consisting of: (A) a mixture comprising an ester-substituted diaryl carbonate, a transesterification catalyst and at least one dihydroxy aromatic compound; and (B) at least one precursor polycarbonate comprising ester-substituted phenoxy endgroups. The extruder is operated at one or more temperatures in a temperature range, at least one of said temperatures being sufficient to promote reaction between hydroxy and carbonate groups present in the starting material, thereby effecting polymer chain growth. The method of the present invention provides the product polycarbonate as an extrudate. The reaction between hydroxy and carbonate groups is advantageously catalyzed by a transesterification catalyst. Where starting material (A) is employed the transesterification catalyst is introduced into the extruder along with the ester-substituted diaryl carbonate and at least one dihydroxy aromatic compound. Where starting material (B) is employed, a transesterification catalyst may be added in addition to the precursor polycarbonate being introduced into the extruder. Frequently, however, the precursor polycarbonate is itself prepared via a melt reaction between an ester-substituted diaryl carbonate and at least one dihydroxy aromatic compound in the presence of a transesterification catalyst. Precursor polycarbonates incorporating ester-substituted phenoxy endgroups may be conveniently prepared by heating a mixture of at least one dihydroxy aromatic compound, such as bisphenol A, with an ester-substituted diaryl carbonate, such as bis-methyl salicyl carbonate, in the presence of transesterification catalyst, such as tetrabutyl phosphonium acetate, at a temperature in a range between 150xc2x0 C. and 200xc2x0 C. and a pressure between about 1 mmHg and about 100 mmHg while removing by-product ester-substituted phenol, said transesterification catalyst being used in an amount corresponding to between about 1xc3x9710xe2x88x928 and 1xc3x9710xe2x88x923 moles catalyst per mole dihydroxy aromatic compound. The transesterification catalysts suitable for use in the melt preparation of precursor polycarbonates comprising ester substituted endgroups include those catalysts described herein. Such transesterification catalysts are reasonably stable under the conditions of the melt preparation of the precursor polycarbonates. Thus, the precursor polycarbonate may contain sufficient residual -transesterification catalyst such that additional transesterification catalyst is often unnecessary to effect substantial molecular weight increase upon extrusion of the precursor polycarbonate. The precursor polycarbonate, starting material (B), may be introduced into the extruder in a variety of forms according to the method of the present invention, including as an amorphous powder, as a partially crystalline powder and as a melt.
The amount of transesterification catalyst present according to the method of the present invention is in a range between about 1xc3x9710xe2x88x928 and about 1xc3x9710xe2x88x923, preferably between about 1xc3x9710xe2x88x927 and about 1xc3x9710xe2x88x923, and still more preferably between about 1xc3x9710xe2x88x926 and about 5xc3x9710xe2x88x924 moles catalyst per mole dihydroxy aromatic compound employed in the case of starting material (A), or in the case of starting material (B) per mole of structural units present in the precursor polycarbonate which are derived from a dihydroxy aromatic compound. The amount of transesterification catalyst present in catalyst systems having multiple components, for example sodium hydroxide and tetrabutyl phosphonium acetate, is expressed as the sum of the number of moles of each component of the catalyst system per mole dihydroxy aromatic compound in the case of starting material (A), or in the case of starting material (B) per mole of structural units present in the precursor polycarbonate which are derived from a dihydroxy aromatic compound.
In one embodiment of the present invention a precursor polycarbonate comprising repeat units V 
is employed as starting material (B), wherein said precursor polycarbonate comprises residual transesterification catalyst, said catalyst being present in an amount such that the mole ratio of transesterification catalyst to bisphenol A-derived structural units V is in a range between about 1xc3x9710xe2x88x928 and about 1xc3x9710xe2x88x923, preferably between about 1xc3x9710xe2x88x927 and about 1xc3x9710xe2x88x923 and still more preferably between about 1xc3x9710xe2x88x926 and about 5xc3x9710xe2x88x924.
Suitable transesterification catalysts according to the method of the present invention include salts of alkaline earth metals, salts of alkali metals, quaternary ammonium compounds, quaternary phosphonium ions, and mixtures thereof. Suitable transesterification catalysts include quaternary ammonium compounds comprising structure VI 
wherein R1-R16 are independently a C1-C20 alkyl radical, C4-C20 cycloalkyl radical or a C4-C20 aryl radical and Xxe2x88x92 is an organic or inorganic anion. Anions Xxe2x88x92 include hydroxide, halide, carboxylate, phenoxide, sulfonate, sulfate, carbonate, and bicarbonate. In one embodiment of the present invention the transesterification catalyst comprises tetramethyl ammonium hydroxide.
Suitable transesterification catalysts include quaternary phosphonium compounds comprising structure VII 
wherein R17-R20 and Xxe2x88x92 are defined as in structure VI. In one embodiment of the present invention the transesterification catalyst comprises tetrabutyl phosphonium acetate.
Where Xxe2x88x92 is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structures VI and VII are properly balanced. For example, where R17-R20 in structure VII are each methyl groups and Xxe2x88x92 is carbonate, it is understood that Xxe2x88x92 represents xc2xd (CO3xe2x88x922).
In one embodiment of the present invention the transesterification catalyst further comprises at least one alkali metal hydroxide, alkaline earth hydroxide or mixture thereof, in addition to a quaternary ammonium compound such as VI, a quaternary phosphonium compound such as VII, or a mixture thereof. Sodium hydroxide in combination with tetrabutyl phosphonium acetate illustrates such mixed catalyst systems. In catalyst systems comprising quaternary xe2x80x9coniumxe2x80x9d compounds such as VI or VII together with a metal hydroxide such as sodium hydroxide, it is frequently preferred that the amount of xe2x80x9coniumxe2x80x9d compound be present in excess relative to the metal hydroxide, preferably in an amount corresponding to from about 10 to about 250 times the amount of metal hydroxide employed.
In one embodiment of the present invention the transesterification catalyst comprises at least one alkali metal salt of a carboxylic acid, an alkaline earth metal salt of a carboxylic acid or a mixture thereof. Salts of ethylene diamine tetracarboxylic acid (EDTA) have been found to be particularly effective, among them Na2Mg EDTA.
In yet another embodiment of the present invention the transesterification catalyst comprises the salt of a non-volatile inorganic acid. By xe2x80x9cnonvolatilexe2x80x9d it is meant that the referenced compounds have no appreciable vapor pressure at ambient temperature and pressure. In particular, these compounds are not volatile at temperatures at which melt polymerizations of polycarbonate are typically conducted. The salts of nonvolatile acids according the present invention are alkali metal salts of phosphites; alkaline earth metal salts of phosphites; alkali metal salts of phosphates; and alkaline earth metal salts of phosphates. Suitable salts of nonvolatile acids include NaH2PO3, NaH2PO4, Na2H2PO3, KH2PO4, CsH2PO4, Cs2H2PO4, and a mixture thereof In one embodiment, the salt of the nonvolatile acid is CsH2PO4. In one embodiment of the present invention the transesterification catalyst comprises both the salt of a non-volatile acid and a basic co-catalyst such as an alkali metal hydroxide. This concept is exemplified by the use of a combination of NaH2PO4 and sodium hydroxide as the transesterification catalyst.
In one embodiment of the present invention, the starting material (A) comprises between about 0.9 and about 1.25, preferably about 0.95 to about 1.05 moles of ester-substituted diaryl carbonate per mole of aromatic dihydroxy compound present in the mixture, and between about 1.0xc3x9710xe2x88x928 to about 1xc3x9710xe2x88x923, preferably between about 1.0xc3x9710xe2x88x926 to about 5xc3x9710xe2x88x924 moles of transesterification catalyst per mole of aromatic dihydroxy compound present in the mixture.
The components of starting material (A); ester-substituted diaryl carbonate, at least one dihydroxy aromatic compound, a transesterification catalyst, and optionally a monofunctional phenol may be introduced into the extruder through the same or separate feed inlets and the rates of introduction of said components and said optional monofunctional phenol may be varied to control the molar ratios of the reactants and in this manner to control the physical properties of the product polycarbonate such as molecular weight and endgroup identity. The method of the present invention thus allows for adjustment of the product polycarbonate molecular weight within the context of a continuous process. For example, a slight adjustment in the relative rates of introduction of ester-substituted diaryl carbonate, dihydroxy aromatic compound and optionally monofunctional phenol may be made during a continuous extrusion operation to vary slightly the molecular weight of the product polycarbonate. Conversely, substantial changes in the product polycarbonate molecular weight may be made, as in for instance a polymer grade change, by more substantial adjustment in the relative rates of introduction of ester-substituted diaryl carbonate, dihydroxy aromatic compound and optionally monofunctional phenol.
The extruder employed according to the method of the present invention is operated at one or more temperatures in a range between about 100xc2x0 C. and about 350xc2x0 C., preferably between about 250xc2x0 C. and about 300xc2x0 C.
The extruder, which may be a single screw or multiple screw extruder is operated at one or more screw speeds in a screw speed range, said range being between about 50 revolutions per minute (rpm) and about 500 rpm, preferably between about 200 rpm and about 400 rpm.
As mentioned, the extruder may be equipped with a vacuum vent. A vacuum vent is necessary in instances where a large amount of ester-substituted phenol by-product is evolved during the extrusion, as in the case wherein starting material (A) is employed. In instances wherein the total amount of by-product ester-substituted phenol is relatively small, as in the case of the extrusion of a precursor polycarbonate comprising ester-substituted phenoxy endgroups IV, said precursor polycarbonate having substantial molecular weight, for example a weight average molecular weight of at least about 16000 Daltons relative to a polycarbonate standard, the use of vacuum vents is optional. In general, however, it is found expedient to practice the method of the present invention on an extruder comprising at least one vacuum vent. Frequently it is preferred to have two or more vacuum vents. In some embodiments of the present invention 4 vacuum vents are employed. More generally, the method of the present invention utilizes an extruder equipped with a sufficient number of vacuum vents to convert the starting material to polycarbonate having the desired molecular weight. The vacuum vents are operated at reduced pressure, usually in a range between about 1 and about 700 mmHg, preferably between about 10 and about 50 mmHg.
Extruders which may be employed according to the method of the present invention include co-rotating, intermeshing double screw extruders; counter-rotating, non-intermeshing double screw extruders; single screw reciprocating extruders, and single screw non-reciprocating extruders.
In one embodiment of the present invention the mixture introduced into the extruder further comprises a chainstopper. The chainstopper may be included with starting material (A) or starting material (B) or a mixture thereof, and can be used to limit the molecular weight of the product polymer or alter its physical properties such as glass transition temperature or static charge carrying properties. Suitable chainstoppers include monofunctional phenols, for example p-cumylphenol; 2,6-xylenol; 4-t-butylphenol; p-cresol; 1-naphthol; 2-naphthol; cardanol; 3,5-di-t-butylphenol, p-nonylphenol; p-octadecylphenol; and phenol. In alternative eembodiments of the present invention the monofunctional phenol may be added at an intermediate stage of the polymerization or after its completion, as where the monofunctional phenol is added downstream of the feed inlet used to introduce starting materials (A) or (B). In such alternative embodiments the chainstopper may exert a controlling effect upon the molecular weight of the product polycarbonate and will control the identity of the polymer terminal groups.
The method of the present invention provides a product polycarbonate having a weight average molecular weight, as determined by gel permeation chromatography, in a range between about 10,000 and about 100,000 Daltons, preferably between about 15,000 and about 60,000 Daltons, and still more preferably between about 15,000 and about 50,000 Daltons; said product polycarbonate having less than about 1000, preferably less than about 500, and still more preferably less than about 100 parts per million (ppm) Fries product. Structure VIH below illustrates the Fries product structure present in a polycarbonate prepared from bisphenol A. As indicated, the Fries product may serve as a site for polymer branching, the wavy lines indicating polymer chain structure. 
Polycarbonates prepared using the method of the present invention may be blended with conventional additives such as heat stabilizers, mold release agents and UV stabilizers and molded into various molded articles such as optical disks, optical lenses, automobile lamp components and the like. Further, the polycarbonates prepared using the method of the present invention may be blended with other polymeric materials, for example, other polycarbonates, polyestercarbonates, polyesters and olefin polymers such as ABS.